The invention relates to an electrolyte or an electrolyte additive which in particular can be used in lithium-ion batteries or other battery designs, such as lithium-air batteries or lithium-sulfur batteries.
Lithium-ion batteries and future battery designs, such as lithium-air batteries or lithium-sulfur batteries apart from a high energy density must fulfill also high demands with respect to safety and reliability which must be ensured for the total life-time. In this regard liquid electrolytes in the past often have emerged negatively, since they have the tendency to dissolve which may lead to a loss in conductivity and/or to undesired disintegration products, and since they are easily flammable.
An alternative are polymeric electrolytes which however have only small ionic conductivities. By contrast, gel-electrolytes, which are a combination of liquid and polymeric electrolytes, often have better conductivities, however have the tendency to be flammable.
Due to this reason it has been tried for a long time to develop solid electrolytes as an alternative. With respect to solid electrolytes an ionic conductivity of at least 10−5 S/cm, better at least 10−4 S/cm is demanded. At the same time the electronic conductivity should be lower by at least 4 to 5 magnitudes to avoid a self-discharging of the battery. In addition the chemical resistance against all materials used in the battery is expected, in particular against metallic lithium. Of course, also a sufficient electrochemical stability during charging and discharging (cycling) of the battery should be present.
Such demands are fulfilled only by few known materials. These include on the one hand sulfidic systems with the main components lithium, phosphorus and sulfur, and on the other hand oxidic systems with NaSiCon or garnet-like crystal phases.
Sulfidic compositions such as Li—S—P, Li2S—B2S3-LI4SiO4 or Li2S—P2S5—P2O5Li—S—P, and Li2S—P2S5—P2O5 often are prepared by grinding the starting ingredients under protective gas and by a subsequent temperature treatment (usually also under protective gas) (confer to this end US 2005/0107239 A1, US 2009/0159839 A1). Partially ionic conductivities of more than 10−3 S/cm are reported at room temperature.
However, the large-scale production of such materials is complicated, since it must be performed in the absence of air, since the materials are not stable under air. In particular, the presence of only small amounts of water has been found to lead to a fast disintegration. This leads to an increase of the production and processing cost and poses a safety-technical problem.
Also with oxidic systems with NaSiCon conductivities of more than 10−4 S/cm can be reached at room temperature.
However, the NaSiCon materials usually are not stable against metallic lithium which requires the utilization of an additional protective layer for protecting the anode (confer EP 1673818 B1). In addition for high conductivities often the use of costly row materials, such as germanium, tantalum or gallium is required.
An alternative are systems with garnet-like crystal phases, such as Li7La3Zr2O12 (US 2010/0047696 A1). From DE 10 2007 030604 A1 and from WO 2005/085138 A1 garnet-like crystal phases (Li7+xAxG3−xZr2O12 (with A: bivalent cation, G: trivalent cation) or Li5+xAxG3−xM2O12 (with A: bivalent cation, G: trivalent cation, M: pentavalent cation) are known. In the pure system Li7La3Zr2O12 at about 150 to 200° C. a reversible phase transition occurs from the tetragonal garnet phase, which is stable at room temperature, to the cubic garnet phase. The tetragonal garnet phase offers a lithium-ion conductivity of about 1.6×10−6 S/cm (confer J. Akawa et al., Journal of Solid State Chemistry 182 (2009), 2046-2052). The cubic phase which can be stabilized by means of dopings, such as with aluminum or tantalum also at room temperature, even shows a conductivity in the order of 10−4 S/cm (confer EP 2 159 867 A1).
Materials with lithium-garnet-like crystal phases usually are prepared by means of a solid body sintering route which often requires several grinding and several temperature treatment operations (confer EP 2 159 867 A1).
A problem with this production are the high temperatures up to about 1250° C. that are necessary for generating the desired crystal phases which at the same time lead to a strong lithium evaporation. The use of a too high lithium proportion leads to a stabilization of the tetragonal phase having poorer conductivity, while a lithium proportion that is too small leads to the generation of foreign phases. This complicates the preparation of highly conductive pure phase material on an industrial scale.
From DE 10 2012 207 424 B3 an ion-conducting, alkaline containing glass ceramic is known wherein after a ceramizing of the glass ceramic from a starting glass at least a part of the alkali-ions of the glass ceramic are exchanged against alkali-ions of a different, preferably smaller, atomic number. The glass ceramic comprises nepheline or carnegeit as main crystal phases and before the alkali-ion-exchange comprises at least the following components: 15-75 wt.-% SiO2, 4-60 wt.-% Al2O3, 4-65 wt.-% Na2O, 0-10 wt.-% Li2O, 0-10 wt.-% TiO2, 0-10 wt.-% ZrO2, 0-5 wt.-% SnO2, 0-20 wt.-% B2O3, 0-30 wt. % P2O5. The glass ceramics are molten within a platinum-rhodium crucible by melting the starting materials at 1600 to 1650° C., refining and stirring, thereafter casting, and are then controlled transferred into a glass ceramic according to a ceramization program. The glass ceramic comprises nepheline (NaAlSiO4) as a dominating phase and as side phases depending on the composition Na2TiSiO5, Li2SiO3, Li2TiO3. In a LiNO3 bath (6 hours at 340° C.) the sodium almost fully is exchanged against lithium. The lithium-ion conductivity is in the range of about 10−4 S/cm. The key part of such a glass ceramic shall be crystal phases of one or more crystal types within which alkali-ions can move relatively free on percolating paths, partially in veritable channel structures within a generated network within fixed bindings of mostly covalent character. It should be noted that such networks in particular can be designed using polyvalent cations, in particular trivalent to pentavalent cations with connecting oxygen atoms. Such crystal types shall be found in many crystal systems, such as in the perovskite-type lithium-lanthanum-titanate La(2/3−x)Li3xTiO3, sodiumbeta-aluminate NaAl11O17, borates, phosphates, chain, band, layer and framework silicates. In this context also the garnet-like Li5La3(Nb, Ta)2O12 is mentioned (confer Philippe Knauth: Inorganic solid Li ion conductors: An overview, Solid State Ionics 180 (2009) 911-916). However, with respect to the preparation nothing is disclosed in this regard.